Can someone share the specs or basic design for a support under the middle of the mast at the mast step for trailering?
Is this support recommended / needed? I'm assuming it is. This will be my first road trip!

I took a treated 3X3 and cut it to the height I want, then epoxied a piece of foam padding to the top. Finally, I drilled a hole for the carriage bolt. This has worked well and allows me to tie my mast down in the middle without fear of bending my mast the wrong way. The benefit big benefit is I can tie up the halyard blocks to keep them off of the deck (one less thing to remove).

I would also note I made collapsible saw horses to support the mast while trailering. I cringe when I see boats with the mast lashed to the the bow pulpit, stern pulpit (if present) or worse yet the pintles. The mast/rigging is pretty heavy (~100lbs) and has a lot of leverage on the deck particularly when you consider it will all be bouncing down the road.